1. Field of the Invention
The present invention relates to an elevator control apparatus and, specifically, to an elevator control apparatus which performs electric current feedback control to drive an induction motor.
2. Description of the Related Art
Hitherto, a system has been known in which a drive and control apparatus, employing a pulse-width-modulation control type inverter, controls an induction motor to vertically move an elevator car.
FIG. 2 shows an example of the construction of a conventional elevator control apparatus employing an inverter of the above-mentioned type. Referring to FIG. 2, the system includes a converter 1 for converting three-phase alternating current (AC) power (comprising R-phase, S-phase and T-phase components) into direct current (DC) power, a smoothing capacitor 2 for smoothing DC output of the converter 1, an inverter 3 for converting DC power smoothed by the smoothing capacitor 2 into three-phase AC power of a variable voltage and a variable frequency, an induction motor 4 driven by AC power from the inverter 3, a sheave 5 connected to the induction motor 4, and a rope 6 wound on the sheave 5 with one end of the rope connected to a counterweight 7, and the other to a car 8. The system further includes current detectors 9 and 10 for detecting phase current of U-phase and v-phase, respectively, for the induction motor 4, a current command generating circuit 11, and operational amplifiers 12, 13 and 14 for U-phase, V-phase and W-phase, respectively, which are connected to the current command generating circuit 11, the operational amplifiers 12 and 13 being connected to receive the respective outputs of the current detectors 9 and 10, and the operational amplifier 14 being connected to receive outputs from the current detectors 9 and 10. The operational amplifiers 12 and 13 include differential amplifiers 12a and 13a, respectively, while the operational amplifier 14 includes differential amplifiers 14a and 14b. A triangular wave generating circuit 15 is provided to generate a triangular wave serving as a carrier wave for pulse-width-modulation control. Comparators 16, 17 and 18 are provided to compare the respective outputs of the operational amplifiers 12, 13 and 14 with the output of the triangular wave generating circuit 15. An inverter stoppage signal generating circuit 19 is provided to generate a signal commanding stoppage of the inverter 3. An inverter control circuit 20 is provided to control the inverter 3 in accordance with the outputs of the comparators 16, 17 and 18.
Next, the operation of the apparatus will be described. When a speed command generating circuit (not shown) generates a speed command, in order that the elevator car 8 will travel in accordance with the command, the inverter stoppage signal from the inverter stoppage signal generating circuit 19 is extinguished, and the current command generating circuit 11 generates current command values I.sub.cu and I.sub.cv of current that should flow to the induction motor 4. The current detectors 9 and 10 detect the current flowing to the induction motor 4, and output the detected current as current feedback signals I.sub.u and I.sub.v. Subsequently, each of the operational amplifiers 12 and 13 operates with respect to U-phase or V-phase to amplify the difference between the current command value I.sub.cu or I.sub.cv, on one hand, and the current feedback signal values I.sub.u or I.sub.v, on the other, and outputs the amplified difference P.sub.wu or P.sub.wv in U- or V-phase. With respect to W-phase, the operational amplifier 14 synthesizes and then amplifies the differences in U-phase and V-phase as between the current command values I.sub.cu and I.sub.cv, on one hand, and the current feedback signal I.sub.u and I.sub.v, on the other, and outputs the resultant difference P.sub.ww in W-phase. Each of the comparators 16, 17 and 18 compares one of the differences P.sub.wu, P.sub.wv and P.sub.ww with triangular wave generated by the triangular wave generating circuit 15, performs pulse-width modulation, and sends the resultant pulse signal to the inverter control circuit 20. In accordance with the pulse signals from the comparators 16, 17 and 18, the inverter control circuit 20 controls a switching element, such as a transistor or insulated-gate bipolar transistor constituting the inverter 3 so that desired AC power is supplied to the induction motor 4. This enables the car 8 of the elevator to ascended or descend in accordance with the speed command.
It is necessary to control the speed of the car 8 in a smooth manner and within a wide range from a start to a subsequent stop. Whether such control is performed or not greatly depends on the accuracy of detection by the current detectors 9 and 10. This is for the following reason: The current detectors 9 and 10 are, as stated before, provided to detect the AC current flowing to the induction motor 4 for driving the elevator car 8. If the outputs of the current detectors 9 and 10 include DC offset voltage, torque ripple is generated due to the offset voltage. The torque ripple may cause vibration of the car 8, making it uncomfortable to ride in the car. The offset voltage varies with variations in the temperature of the current detectors 9 and 10 or variations thereof caused by the passage of time. Therefore, adjusting the offset voltage to zero when shipping the current detectors as products does not makes it possible to prevent generation of offset voltage during operation.
Conventional efforts to overcome the above-described problem include the art disclosed, e.g., in Japanese Patent Laid-Open No. 60-23268. FIGS. 3 and 4 show different arrangements disclosed in the above document. These figures each show only that part of the arrangement related to the U-phase because the remaining part is the same as that shown in FIG. 2, and because the construction is the same with respect to all of the U-, V- and W-phases. The arrangement shown in FIG. 3 includes an offset voltage cancellation circuit 21 having a sample-and-hold circuit 21a for sampling and holing outputs of the operational amplifier 12 in response to an inverter stoppage signal, a contact 21b for disconnecting the output of the sample-and-hold circuit 21a, and a capacitor 21c for allowing the sample-and-hold circuit 21a to maintain a voltage. The arrangement shown in FIG. 4 includes an offset voltage cancellation circuit 22 having an A/D converter 22a for converting an analog signal from the operational amplifier 12 into a corresponding digital signal, a memory 22b for storing the digital signal output by the A/D converter 22a, and a D/A converter 22c for converting the digital signal from the memory 22b into a corresponding analog signal, the arrangement shown in FIG. 4 also includes a main control circuit 23 for controlling, in accordance with an inverter stoppage signal, the A/D converter 22a, the memory 22b and the D/A converter 22c.
The arrangement shown in FIG. 3 provides the following operation. When the elevator car 8 has stopped, and the inverter stoppage signal generating circuit 19 generates an inverter stoppage signal, the sample-and-hold circuit 21a starts sampling the output P.sub.wu of the operational amplifier 12. At this time, the current command generating circuit 11 is controlled in such a manner that the current command value I.sub.cu, serving as the input to the operational amplifier 12, becomes zero. Consequently, the inverter 3 is stopped and no current flows to the induction motor 4 so that the current feedback signal I.sub.u from the current detector 9 represents only the offset voltage of the current detector 9. Further, the contact 21b is opened so that no signal is applied from the sample-and-hold circuit 21a to the operational amplifier 12. As a result, only the offset voltage of the current detector 9 is, after being amplified by the differential amplifier 12a, output at the output terminal of the operational amplifier 12, which output is stored by the sample-and-hold circuit 21a. Thereafter, when the inverter stoppage signal is extinguished in order to start the car 8, the sample-and-hold circuit 21a is brought into its holding state, in which the output P.sub.wu of the operational amplifier 12 sampled during the stop period of the car 8 is held. The contact 21b is closed so that the output P.sub.wu stored by the sample-and-hold circuit 21a is supplied to the operational amplifier 12, whereby the offset voltage of the current detector 9 is cancelled.
With the arrangement shown in FIG. 4, when the elevator car 8 has stopped, and the inverter stoppage signal generating circuit 19 generates an inverter stoppage signal, the main control circuit 23 causes the output P.sub.wu of the operational amplifier 12 to be supplied through the A/D converter 22a to the memory 22b to be stored therein. At this time, similarly to the case shown in FIG. 3, the current command generating circuit 11 is controlled in such a manner that the current command value I.sub.cu serving as the input to the operational amplifier 12, becomes zero. Consequently, the current feedback signal I.sub.u from the current detector 9 represents only the offset voltage of the current detector 9, and the control circuit 23 operates to output zero at the output terminal of the D/A converter 22c. As a result, only the offset voltage of the current detector 9 is, after being amplified by the differential amplifier 12a, output at the output terminal of the operational amplifier 12, and this output is stored into the memory 22b through the A/D converter 22a. Thereafter, when the inverter stoppage signal is extinguished to start the car 8, the main control circuit 23 operates to cause the output P.sub.wu of the operational amplifier 12 stored in the memory 22b during the stop period of the car 8 to be supplied through the D/A converter 22c to the operational amplifier 12. In this way, the offset voltage of the current detector 9 is cancelled, thereby enabling torque ripple to be restrained from being generated in the induction motor 4 due to the offset voltage.
A conventional elevator control apparatus, such as that described above, is arranged such that each offset voltage cancellation circuit stores the output of an operational amplifier. Accordingly, it is necessary that those inputs to the operational amplifiers other than the offset voltage of the current detectors be made zero. However, this requirement makes it impossible to keep outputting the current command even during the stoppage period of the inverter in order to detect faults in the inverter stoppage signal generating circuit (as shown, e.g., in Japanese Patent Laid-Open No. 60-228378), which is disadvantageous from the viewpoint of safety.
Since the conventional arrangement shown in FIG. 4, in which the offset voltage of the current detectors is sampled, requires the inclusion of elements such as an A/D converter and a memory, the entire apparatus is relatively expensive, which is another disadvantage.